Certain pumps, such as those for oil field use, are typically designed in two sections, the (proximal) power section (herein “power end”) and the (distal) fluid section (herein “fluid end”). The power end usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. The fluid end of such a pump comprises a housing which in turn comprises one or more functional units, each functional unit comprising a suction valve, a discharge valve, a bore or cylinder, and a piston (e.g., in mud pumps) or plunger (e.g., in fracking pumps), plus packing assemblies, high-pressure seals, etc.
Conventional plunger pumps employ a housing member (i.e., a packing box) containing plunger packing assemblies analogous to packing assemblies shown in U.S. Pat. No. 4,572,519, incorporated herein by reference and referred to subsequently as the '519 patent. Within a packing assembly one or more packing rings having corresponding chevron-shaped ends are retained and compressed between a proximal brass (or bronze) adapter ring and a distal adapter ring.
The packing rings have chevron-shaped ends (sometimes termed “chevron-shaped packing rings”) and are relatively stiff, comprising, for example, layered fabric-reinforced rubber. Each ring end's chevron shape arises from a concave or convex circumferential feature which is an extension of their layered construction, allowing modest radial expansion under longitudinal compression to achieve a tight seal around a plunger. Longitudinal compression force, in turn, is provided by a gland nut and by the cyclically increased pressure of the pumped fluid.
Turning the gland nut adjusts longitudinal preload (i.e., longitudinal compressive force) that is applied to the packing ring(s) to prevent excessive cyclic movement under load and to achieve a desired seal around the plunger. But even after reducing longitudinal preload to zero by backing out the gland nut, packing rings typically remain tightly sealed around the plunger because of their stiffness. Thus, the plunger must virtually always be withdrawn from the packing box to allow removal of the packing ring(s), as may be required during pump maintenance. Plunger withdrawal, however, is generally difficult because of interference between the plunger and various power end components.
The above maintenance issues arise in conventional plunger pumps because designers make the packing rings relatively stiff, while still flexible enough under longitudinal compression to achieve a tight seal around the plunger. Sealing against fluid leakage, however, requires that the packing rings substantially retain their functional (chevron) end shape. Such shape retention becomes progressively more difficult as frictional heat developed during pump operation softens the packing rings and predisposes them to extrusion under pressure through a gap (the extrusion gap) between the proximal adapter ring and the plunger.
A conflict thus arises when packing ring compression is increased. Increased compression may help to improve the plunger seal as well as to reduce migration of the packing ring material through the extrusion gap. But increased compression of the packing rings also results in generation of additional frictional heating that degrades the rings' functional integrity. Since dissipating this heat within the confines of the packing box is difficult, high pressure plunger pumps are often limited to relatively short periods of operation (e.g., no more than a few hours) to limit extrusion of the packing ring material through the extrusion gap. If pump run times are extended, packing rings tend to overheat and fail prematurely.
The above conflict has been partially resolved by replacing the proximal bronze adapter ring with a proximal anti-extrusion adapter ring comprising PEEK polymer in certain seals. PEEK (polyetheretherketone) is a high performance thermoplastic, a portion of which moves radially inward under longitudinal compression, tending to narrow the extrusion gap. See, e.g., U.S. Pat. No. 7,847,057 B2, incorporated by reference. Such radial inward (gap-narrowing) movement is termed “elongation” in industry advertising and is said to be more prominent in a PEEK adapter ring than in an analogous bronze adapter ring. While the use of commercially-available PEEK adapter rings appears to increase plunger packing service life, frictional wear and heat generation problems remain.
Additional remaining problems relate to the confounding effects of the time-varying character of pumped-fluid pressure. One pressure change in particular occurs as the suction valve quickly moves from full-open to closed (i.e., as a pump pressure stroke begins). Fluid flow through the valve changes direction momentarily and then stops abruptly. Since the pumped fluid is substantially incompressible, a high-amplitude fluid pressure transient is generated and virtually instantaneously transmitted from the suction valve to, among other sites, the plunger seal. This closing energy impulse can excite damaging vibration resonances in a pump while increasing the risk of extrusion damage to a conventional seal. But the vibration can be damped and the risk of extrusion damage can be reduced with a new approach to design and operation of the anti-extrusion member.